Natural gas is a high quality and clean petrochemical energy, with a quite important position in national economy. The liquefaction and storage of natural gas is a critical technology in its development and utilization, it has become an industry both at home and overseas, and grows at an average annual rate of 8%, in recent years, it has been growing quite rapidly in the energy consumption pattern of China. The technology to liquefy natural gas has become a high technology, being attached with importance by more and more scientific and technological sectors.
It is expected that by the mid of this century, if China consumes natural gas of 5000*108 m3/a, including import of LNG1000*108 m3/a (equivalent the present import of Japan), the usable cold energy is 257*108 kWh/a, equivalent to the annual power generation amount of a power plant of 600*104 kW. Therefore, it is worth our in-depth consideration on how to realize breakthroughs in aspects of technology, management mechanism and market operation for LNG, to substantially reduce the energy consumption for LNG, push forward rapid development of large scale cold energy industrial chain including air separation and gasification of coal with enriched air for huge energy conservation and economic benefit, so as to make contribution to the full realization of cyclic economy and saving economy in China. In the meantime, the rapid development and transition of Chinese economy has determined the absolute necessity of large-scale use of LNG, and also provided a huge user market.
Traditional natural gas liquefaction is mainly based on the following three processes:
1. Cascade liquefying process (also referred to as step liquefaction process, overlapped liquefying process or serial vaporization and condensation liquefying process), mainly applied in natural gas liquefying apparatus carrying basic loads;
2. Mixed refrigerant liquefying process: the so-called MRC liquefying process, is a process in which a medium of mixed refrigerant with multi-components including hydrocarbon compound of C1 to C5 and N2, is condensed, vaporized and throttle expanded to obtain a certain refrigerating capacity at different temperature level so as to refrigerate and liquefy natural gas step by step. MRC has achieved the purpose similar to cascade liquefying process, and also overcome its disadvantage of complicated system. Since the 1980s, almost all newly built and expanded natural gas liquefaction apparatus for basic loads are based on the liquefying process with propane precooling mixed refrigerant;
3. Liquefying process with expander: this process is based on the Claude cycle of refrigerant in a turbine expander, to realize liquefaction of natural gas. When the gas expands and makes work in an expander, the temperature is lowered and power recovered. Depending on different refrigerant, it can be classified as nitrogen expansion liquefying process and natural gas expansion liquefying process. These processes have the advantages: (1) simple process, flexible regulation, reliable working, easy startup and operation, and convenient maintenance; and (2) with the natural gas itself used as medium, it can save the expense of production, transport and storage of refrigerant. The disadvantages are: (1) all gas streams to the apparatus requires in-depth drying; (2) the reflux pressure is low, the area of heat exchange is large and the input of equipment metal is high; (3) it is restricted by the number of LP users; (4) the liquefaction rate is low, if re-circulation is required, the power consumption will increase greatly when additional circulation compressors are used. A liquefying process with expander is easy to operate with moderate investment, and is particularly suitable to peak regulation type natural gas liquefaction apparatus with fairly low capacity.
FIG. 1 is a schematic diagram of cascade natural gas liquefying process.
FIG. 2 is a schematic diagram of APCI propane precooling mixed refrigerant liquefying process.
FIG. 3 shows the natural gas expansion liquefying process, in which: 301—dehydrating agent, 302—carbon dioxide removal column, 303—water cooler, 304—returns to the gas compressor, 305, 306, 307—heat exchangers, 308—subcooler, 309—tank, 310—expander, 311—compressor.
FIG. 4 shows the nitrogen expansion liquefying process, in which: 401—pre-treatment apparatus, 402, 404, 405—heat exchanger, 403—heavy hydrocarbon separator, 406—nitrogen stripper, 407—turbine expander, 408—nitrogen-methane separating column, 409—circulation compressor.
FIG. 5 is a schematic diagram of natural gas expansion liquefying process with propane precooling, in which: 401, 403, 405, 406, 407—heat exchangers, 402, 404—propane heat exchangers, 408—water cooler, 409—compressor, 410—braking compressor, 412, 413, 414—gas-liquid separator.
The design of the afore-said traditional natural gas liquefying processes is mainly based on the theoretical foundation of thermodynamics, Carnot reverse cycle of identical temperature difference is used to analyze the natural gas liquefying process, the economic indicator of the cycle is the refrigeration coefficient, or the ratio of obtained gain to the cost of consumption, and also, of all refrigerating cycles between atmospheric environment with temperature of T0 and low temperature heat source with temperature of Tc (such as refrigeration store), the reverse Carnot cycle has the highest refrigeration coefficient:
                              ɛ          c                =                                            (              COP              )                                      R              ,              C                                =                                                    q                2                                            w                0                                      =                                          T                c                                                              T                  0                                -                                  T                  c                                                                                        (        1        )            
In the formula above, ϵc is the refrigeration coefficient, q2 refrigerating capacity of the cycle, and w0 the net work consumed by the cycle.
The actual cycle efficiency is usually described by the ratio of refrigeration coefficient of actual cycle and theoretical cycling refrigeration coefficient, however, its theoretical basis is cyclic analysis of refrigerating process with Carnot reverse cycle.
In fact, in his thesis “Reflections on the Motive Power of Heat”, Carnot concluded that: of all heat engines working between two constant temperature heat sources of different temperatures, the reversible heat engine has the highest efficiency.” This was later referred to as the Carnot theorem, after rearranging with the ideal gas state equation, the thermal efficiency of Carnot cycle obtained is:
                              η          c                =                  1          -                                    T              2                                      T              1                                                          (        2        )            
In Formula (2), temperature T1 of the high temperature heat source and temperature T2 of low temperature heat source are both higher than the atmosphere ambient temperature T0, and the following important conclusions can be obtained:
1) The thermal efficiency of Carnot cycle only depends on the temperature of high temperature heat source and low temperature heat source, or the temperature at which the media absorbs heat and release heat, therefore the thermal efficiency can be increased by increasing T1 and T2.
2) The thermal efficiency of Carnot cycle can only be less than 1, and can never be equal to 1, because it is not possible to realize T1=∞ or T2=0. This means that a cyclic engine, even under an ideal condition, cannot convert all thermal energy into mechanical energy, of course, it is even less possible that the thermal efficiency is greater than 1.
3) When T1=T2, the thermal efficiency of the cycle is equal to 0, it indicates that in a system of balanced temperature, it is not possible to convert heat energy into mechanical energy, heat energy can produce power only with a certain temperature difference as a thermodynamic condition, therefore it has verified that it is not possible to build a machine to make continuous power with a single heat source, or the perpetual motion machine of the second kind does not exist.
4) Carnot cycle and its thermal efficiency formula are of important significance in the development of thermodynamics. First, it laid the theoretical foundation for the second law of thermodynamics; secondly, the research of Carnot cycle made clear the direction to raise the efficiency of various heat power engines, i.e. increasing the heat absorbing temperature of media and lowering the heat release temperature of media as much as possible, so that the heat is release at the lowest temperature that can be naturally obtained, or at the atmospheric temperature. The method mentioned in Carnot cycle to increase the gas heat absorbing temperature by adiabatic compression is still a general practice in heat engines with gas as media today.
5) The limit point of Carnot cycle is atmospheric ambient temperature, and for refrigerating process cycles below ambient temperature, Carnot cycle has provided no definite answer.
However, the basic theory of thermodynamics cannot make simple, clear and intuitional explanation of the cycling process of natural gas liquefying apparatus, to produce 1 ton of LNG, the power consumption of equipment and utilities is about 850 kWh, which means very high energy consumption in the process.
Einstein commented the classical thermodynamics this way: “A theory will give deeper impression to the people with simpler prerequisite, more involvement and wider scope of application.” In the exploration of basic theory in the refrigeration field, this point should be inherited and carried forward.
Therefore, it has become a difficult issue in the research of natural gas liquefaction technical field to research on the natural gas liquefaction cycles, to really find the theoretical foundation for the refrigerating apparatus cycle and the correct direction to improve the process, and to organize new natural gas liquefying apparatus process on this foundation and substantially reduce the energy consumption of natural gas liquefying apparatus.